When I was doing this exercise with a friend on Understudy, he found solutions to multiplication and exponentiation too. I'm still trying to wrap my head around the exponentiation. Passing a number to another number?!

  (define (multiply number1 number2)
    (lambda (f)
      (number1 (lambda (x) ((number2 f) x)))))
  
  (define (exponentiate base exponent)
    (exponent base))